In general, the double-bond isomerization reaction of olefins often occurs in the presence of an acidic catalyst by proton transfer between olefins. However, it is not necessary to proceed with the reaction at high temperature. Skeletal isomerization of olefins is known to be conducted by contacting unbranched olefins with acidic catalysts at more extreme conditions. Therefore, for skeletal isomerization of olefins, the alkyl carbonium ion which is the transition state of double-bond isomerization reaction needs to undergo a rearrangement, and subsequently a proton transfer step to form the desired products.
In the past, the skeletal isomerization catalysts were mostly prepared from an alumina support with some modifications on the surface by halogen-containing compounds, such as HBr or butyl bromide. Examples are: Sun U.S. Pat. No. 4,778,943, issued Oct. 18, 1988; Sun U.S. Pat. No. 4,654,463, issued Mar 31, 1987; and Elazar et al U.S. Pat. No. 4,433,191, issued Feb. 21, 1984. The catalyst in this invention is prepared without any modification of halide compound on the catalyst surface and can be effectively employed for the skeletal isomerization of olefins.
The isomerization reaction of olefins is well known to be limited by the thermodynamic equilibrium of reacting species. The skeletal isomerization catalysts described in some patents or published papers were mostly prepared from the treatment of halogen-containing compounds in order to maintain the activity and the selectivity. However, there are some disadvantages for those catalysts, such as excessively high capital costs, undue waste, difficulty of treatment, etc.
The product of skeletal isomerization for C.sub.4 olefin is isobutylene which is one of the feedstock reagents for producing methyl tertiary butyl ether (MTBE), an ether compound with high octane number used in gasoline. For the skeletal isomerization of C.sub.5 olefin, the desired product is isoamylene which is the feedstock reagent for producing tertiary amyl methyl ether (TAME), also a high octane-number compound.